Double-headed spring contact probe assembly

ABSTRACT

A double-headed spring contact probe for loaded board testing including a barrel having a hollow interior and opposite plungers which slide axially in the barrel is described. The plungers have outer portions which extend through the opposite open ends of the barrel, each terminating in a contact tip outside the barrel for contacting a test point on a circuit board. One of the plungers has a hollow receptacle extending into the barrel with a rectangular or notched keyway opening into the receptacle. The keyway is disposed at an angle of about 6° to the longitudinal axis thereof. The other plunger has a twisted guide member extending through the barrel into the keyway of the other plunger whereby axial translation of the plungers relative to each other causes a rotation thereof and the keyway rides in the groove for electrical contact as translational movement occurs. A leafspring is further provided on the hollow receptacle adjacent the notched keyway for contacting the twisted guide member of the other plunger to minimize electrical resistance during axial translation. In addition, the plunger may have an open end extending from the barrel with inwardly directed fingers for engaging a contact tip for retaining the same therein.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of patent application Ser.No. 07/848,894, filed Mar. 10, 1992, now U.S. Pat. No. 5,227,718.

TECHNICAL FIELD

The invention relates to electrical probes and, more particularly, tominiature spring-loaded probes for providing electrical contact betweenelectrical components mounted on parallel circuit boards.

BACKGROUND ART

Testing, diagnosis, maintenance and calibration of electronic devicesoften require supplying test signals to, and receiving signals from,components of a Device Under Test (DUT) or Unit Under Test (UUT). Whenan electronic device is fabricated on one or more circuit boards,electronic components mounted on the circuit boards may not beaccessible for testing using existing circuit board mounted connectors.Therefore, connections to components to be tested are made usingexternal electrical probes applied to the exposed leads of thecomponents and/or to a printed circuit board wiring layer.

Automatic testing of electrical circuits requires simultaneousconnection to many circuit test points. The automatic testing equipmentsimultaneously supplies signals to, and receives signals from,combinations of test points. A conventional test fixture used toelectrically probe a circuit card of a DUT includes a "bed of nails"having a platform for supporting the circuit card and an array of singleheaded spring probes. Each spring probe includes a probe head whichmakes positive electrical contact with an overlying portion of thecircuit board being tested. An opposite end of each probe is connectedto test equipment through single point wiring.

A conventional single headed electrical test probe is described byJohnston et al., U.S. Pat. No. 5,032,787 issued Jul. 16, 1991,incorporated herein by reference. The Patent describes a test probeassembly including a barrel having a hollow interior and a plunger whichslides axially in the barrel. The plunger has an outer portion extendingthrough an open end of the barrel, terminating in a contact tip outsidethe barrel for contact with a test point and a hollow, elongatedreceptacle extending through the barrel. The receptacle has a square orrectangular pilot hole so that an elongated fixed guide member in thebarrel extends through the pilot hole. The guide member extends throughthe interior of the barrel away from the pilot hole and has an outersurface which engages the pilot hole. A spring inside the barrel extendsalong the guide member and is biased against the internal end of thereceptacle inside the barrel.

Axial travel of the Johnston et al. plunger into the barrel is againstthe spring bias. The outer surface of the guide member engages thecorrespondingly shaped pilot hole and controls rotational motion of theplunger as it travels along the guide member against the bias of thespring. Thus, the Johnston et al. probe is useful to connect a test leadto a component or conductive layer on a circuit board.

In testing equipment and other electronic equipment which must beadapted to varied uses, it is often necessary to reconfigure signalconnections and condition signals to interface the equipment to aparticular DUT. This can be accomplished by dedicated wiring, patchpanels, and/or using appropriate signal routing/conditioning interfaceequipment in the form of a personality board. A personality board isconnected between a testing device and a DUT to properly route andcondition signals between the two devices. Thus, a testing device iselectrically adapted to a particular DUT by using an appropriatepersonality board. Substitution of personality boards allows a singletesting device to be used with a plurality of DUTs.

The testing device is connected to a personality board which, in turn,is connected to a test fixture holding the DUT using conventionalelectrical connectors and cabling. However, the additional wiring usedto connect the personality board can impair signal connectivity anddegrade the transmitted signals. The added connectors and cables alsoincrease device cost and require additional mounting space on eachcircuit board and between circuit boards. Further, the device connectorsare subject to misalignment and introduce maintenance and reliabilityproblems. Multiple connectors and cabling also complicate thesubstitution of personality boards.

Accordingly, a need exists for a connector system providing easyinstallation and replacement of circuit board mounted devices.

A need further exists for a low resistance electrical connector forinterfacing circuits and wiring mounted on opposing circuit boards.

A still further need exists for a reconfigurable connector system forinterfacing various nodes of an electronic device to a correspondingpoint of a second electronic device without intervening connectors.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a connector and connectorsystem permitting ready installation and replacement of circuit boardsrequiring frequent changing.

Another object of the invention is to provide a connector and connectorsystem for directly connecting electronic circuitry on opposing parallelcircuit cards.

Another object of the invention is to provide a low loss signal pathbetween electronic devices.

A further object of the invention is to provide a universal array ofconnectors for electrically interfacing a variety of electronic devices.

According to one aspect of the invention, a double-headed spring contactprobe for loaded board testing includes a barrel having a hollowinterior and opposite plungers which slide axially and are free torotate, in the barrel. The plungers have outer portions which extendthrough opposite open ends of the barrel, each terminating in a contacttip outside the barrel for contacting a test point on a circuit board.One of the plungers has a hollow receptacle extending into the barrelwith a rectangular or notched keyway opening into the receptacle. Theother plunger has a twisted guide member extending through the barrelinto the keyway of the other plunger whereby axial translation of theplungers relative to each other causes relative rotation thereof. Aspring engages opposite shoulder or collar portions of the plungers tobias the plungers outwardly against opposite ends of the barrel. Neckedportions of the barrel limit travel of the plungers out from the barrel.

Rotation of the probe ends improve the resultant contact of the probewith the circuit board under test or a companion personality board asthe rotating ends sweep oxide off of the contact area. The inventionalso exhibits improved electrical conductivity between plungers byproviding a straight line current path between plungers, conductivitybetween the plungers provided by engagement of the guide member at thekeyway opening. The double-headed construction further accommodatesdirect Printed Circuit (PC) board to PC board electrical connection.

According to another aspect of the invention, an electrical probeincludes a housing having first and second opposite open ends. A firstelongate plunger is partially positioned within the housing. A firstportion of the first plunger extends out from the housing through thefirst opening end and terminates in a first electrical contact probe. Asecond portion of the first elongate plunger is rod-like, and islengthwise contiguous with the first portion. The second portion ispositioned within the housing and includes a bearing surface fortransmitting a torque.

A second elongate plunger is likewise partially positioned within thehousing. A first portion extends out from the housing through the secondopen end of the housing and terminates in a second electrical contactprobe head. The second portion of the second elongate plunger istubular, lengthwise contiguous with the first portion, and is positionedwithin the housing. The second rod-like portion of the first elongateplunger is centrally positioned within the hollow second portion of thesecond elongate plunger. An aperture within the second portion of thesecond elongate plunger engages the bearing surface of the firstelongate plunger.

A compression spring is positioned within the housing and is positionedto engage the first and second plungers, biasing the plungers outwardfrom the housing. The ends of the housing have restricted apertures forlimiting axial travel of the first and second elongate plungers out fromthe housing.

According to a feature of the invention, one or more spiral channels areformed along a length of the second portion of the first elongateplunger, the bearing surface being formed by one or more walls of one ormore channels. The aperture in the second elongate member may comprise akeyway and the second portion of the first elongate plunger may comprisea cylindrical member having a helical channel forming the bearingsurface for engaging the keyway. The helical channel may subtend aradial angle of between 90 and 150 degrees over the length of the secondportion of the first elongate plunger, an angle of 120 degrees plus orminus five degrees being preferred.

According to still another aspect of the invention, an electricalinterface board includes a frame made of an insulating substrate and anarray of probe assemblies extending axially through the frame. Each ofthe probe assemblies includes a housing having first and second oppositeopen ends, a spring and first and second plungers. The first plunger ispartially positioned within the housing and has a first portionextending out from the housing through the first opening end andterminating in a first electrical contact probe. A rod-like secondportion of the first plunger is lengthwise contiguous with the firstportion and is positioned within the housing. The second portion has abearing surface for transmitting a torque.

A second plunger is also partially positioned within the housing andincludes a first portion extending out from the housing through thesecond open end, terminating in a second electrical contact probe head.A tubular second portion, lengthwise contiguous with the first portion,is positioned within the housing so that the second portion of the firstplunger is positioned within the hollow cavity of the second portion ofthe second plunger. The second portion of the second plunger has anaperture at a terminal end for engaging the bearing surface of the firstelongate plunger. The compression spring is positioned within thehousing and engages the first and second elongate plungers, axiallybiasing the plungers outward from the housing.

In a further improvement or additional embodiment of this invention, theplunger barrel portion is designed for increased electrical contact withthe plunger and its spiral groove. Specifically, in this embodiment, thekeyway for engaging the plunger channel is disposed at about a 6° angleto the axis thereof so that the keyway will ride in the plunger channel.In addition, a leafspring is disposed along the keyway on the barrel tocontact the plunger as it translates along the length of the barrel.Finally, the plunger barrel portion which normally receives a solidprobe may have mutually spaced leaves inwardly crimped from the endthereof to engage the probe and retain the same in the barrel plunger.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cross sectional view of a double-headed twist probeaccording to the invention.

FIG. 2 is a sectional view of a probe housing prior to insertion ofplungers and crimping of the open insertion end.

FIG. 3 is a partial sectional view of a plunger barrel portion.

FIG. 4 is a partial sectional view of a barrel keyway.

FIG. 5 is a partial cross-sectional view of a plunger barrel.

FIG. 6 is a side view of a plunger prior to twisting.

FIG. 7 is a side view of a plunger after twisting to form a spiralchannel.

FIG. 8 is a partial sectional view of a terminal end of a plungerforming an electrical contact probe head.

FIG. 9 is a cross-sectional view of a plunger showing channel detail.

FIG. 10 is a partial sectional view of an alternate embodiment of adouble-headed twist probe.

FIG. 11 is a partial sectional side view of a Twin Access Connector(TAC) module including an array of double-headed twist probes providingelectrical connectivity between components of a test device.

FIG. 12 is a partial sectional view of double-headed twist probe mountedin a module.

FIG. 13 is a partial sectional view of wireless fixture for interfacinga personality board to a printed circuit board under test.

FIG. 14 is a partial sectional view of the wireless fixture shown inFIG. 13 with the printed circuit board positioned to engage the twistprobes.

FIG. 15 is a partial sectional view similar to FIG. 3 illustrating analternative embodiment of the plunger barrel portion.

FIG. 16 is a view taken along lines 16--16 of FIG. 15.

FIG. 17 is a view taken along lines 17--17 of FIG. 15; and

FIG. 18 is a view taken along lines 18--18 of FIG. 15.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a miniature double-headed twist probe 20 includes ahollow tubular housing 22 having open ends 23 and 24. A tubular plunger30 is slidably positioned within central cavity 25 of housing 22,extending outward through aperture 23 and terminating in contact tip 38.An internal barrel portion 34 of plunger 30 is coaxial with housing 22,extending approximately to the midpoint of the housing. Opposingrod-like plunger 50 is slidably positioned within an opposite portion ofcavity 25, extending out from housing 22 through aperture 24 andterminating in contact tip 60. An internal twisted rod portion 54 ofplunger 50 is shaped like a drill bit or slotted helix, extendingthrough a matching aperture or keyway 44 of barrel portion 34. Bothplungers 30 and 50 are free to rotate and longitudinally translatewithin housing 22. External portions of plungers 30 and 50 are made of aconductive substance such as heat treated beryllium copper (BeCu) orhardened steel plated with gold over nickel. Housing 22 is preferablymade of a material such as deep drawn gold plated brass or nickelsilver.

Plunger 30 includes lengthwise contiguous internal hollow tubular orbarrel portion 34 and external probe portion 36 which axially extendsout through an aperture in an end of housing 22. A shoulder portion 42limits travel of plunger 30, maintaining the probe within housing 22 byengaging a restricted portion of the aperture formed by crimping orrolling. An inner face of shoulder portion 42 serves as a seat forspring 70 which biases plunger 30 outward from housing 22.

Plunger 50 axially extends through an opposite aperture in housing 22and includes an internal twist rod portion 54 within the housing and anexternal probe portion 58 having a terminal contact tip 60. Internaltwist rod 54 is helically formed and includes a twisted bearing surface56. Internal twist rod 54 passes through an aperture forming a keyway 44in an internal terminal end of barrel 34. Keyway 44 engages twist rod54, including bearing surfaces 56 thereof so that axial travel of theplungers results in relative rotation thereof.

Spring 70 is positioned within cavity 25 of housing 22, coaxiallysurrounding barrel 34 and twisted rod 54 of the plungers. Spring 70 ismade of a spring material such as stainless steel, music wire orberyllium copper and is positioned within housing 22. Opposite ends ofspring 70 are seated on and engage shoulder portion 42 and collarportion 62 of plungers 30 and 50, respectively, thereby biasing theplungers outward from the housing. Inward travel of plungers 30 and 50is against an outward bias provided by spring 70.

Housing 22 prior to assembly of the twist probe is shown in FIG. 2 ofthe drawings. The housing has a substantially tubular body with anaperture 24 formed at one end while an opposite end 26 remains open forinsertion of the remaining probe components. A bulge in the housingforms press ring 28 for retaining the twist probe housing in a supportmember. After plunger 30, spring 70 and probe 50 are inserted intohousing 22, open end 26 is rolled to form a lip, securing the componentswithin the housing.

Plunger 30 is shown in greater detail in FIGS. 3-5 of the drawings.Therein, plunger 30 is made of a tubular material such as 360 brassplated with gold over nickel. Plunger 30 has an open internal barrelportion 34 and a closed external probe portion 36 terminating in contacttip 38. Shoulder portion 42 both limits axial travel of the probe withinhousing 22, and provides a seating surface for engaging spring 70,biasing plunger 30 outward from housing 22. Keyway 44 includes crimpedportions 46 forming tabs extending radially into the cavity 34 ofinternal barrel 32. These tabs are configured to engage bearing surface56 of probe 50.

Referring to FIGS. 6-9, plunger 50 includes an internal rod portionforming internal twist rod 54. Initially, as shown in FIG. 6, a straightchannel 64a is formed in opposite sides of the surface of twist rod 54.The rod is then twisted 120 degrees as shown in FIG. 7 so that a spiralgroove is formed by the twisted channel 64b. Channel 64b is configuredto engage keyway 44 of plunger 30 whereby relative axial movement of theprobes causes relative rotation of the probes. Collar portion 62 ofplunger 50 limits axial travel of the probe and forms a seat for theopposite end of spring 70, biasing probe 50 outward of housing 22against plunger 30. Shoulder portion 63 abuts collar portion 62 on theinner portion of plunger 50 and engages an inner surface of spring 70 tomaintain coaxial alignment of the spring within housing 22.

Although the keyway and matching bearing surface of plungers 30 and 50are shown as inward protruding tabs or "divots" engaging a channel,other geometric shapes can be used. For example, keyway 44 may comprisea rectangular aperture to engage a plunger having a corresponding matingrectangular cross-section. The tab/channel combination, however, has theadvantage of increasing plunger-to-plunger contact surface area therebyminimizing electrical resistance through the probe.

A further embodiment of the plunger barrel shown in FIGS. 3-5 isprovided in FIGS. 15-18. In that embodiment, the plunger barrel 30'mounts the inwardly protruding tab or "divot" 44'. However, it isdisposed at an acute angle of about a 6° angle to the longitudinal axisof the plunger barrel 30' (or an obtuse angle if viewed in the oppositedirection of 174°) to more precisely accommodate the spiral grooveformed by the twisted channel 64b in twist rod 54. As will be obvious tothose skilled in the art, by providing the keyway 44' at the angleshown, it will engage channel 64b for better electrical connection. Asshown in FIG. 16, the keyway 44' is provided on opposite sides of thebarrel portion 34 to engage both the grooves 64b in the twist rod 54. Inthis way, the key way 44' will engage the channels and continue toengage the channels during relative rotational movement of the plungerscaused by relative movement thereof along the common longitudinal axis.

In addition, with attention to FIGS. 15 and 18, the barrel portion 34also provides a leafspring 47 having a bearing surface 47' forcontacting the plunger twist rod 54 as it moves axially through thebarrel 30'. As shown in FIG. 18, the leafspring 47 is stamped from thebarrel portion and is integral therewith.

In another embodiment of the invention, spring 22 can be positionedwithin internal barrel 34 to bias plungers 30 and 50 axially outwardfrom housing 22. An alternate construction of the double-headed twistprobe is illustrated in FIG. 10. Housing 22 and plunger 50 aresubstantially the same as in the first embodiment of FIG. 1. However,plunger 30a is constructed of discrete portions including a barrelportion 34a having a distal end including collar portion 42a retaining asolid probe 36a. Although this embodiment requires more machining thanis required by the first embodiment, the discrete solid probe 36aaccommodates a greater variety of geometries for contact tip 38a.

As an alternative to collar portion 42a, as shown in FIGS. 15 and 17,leaves 43 may be provided around the circumference of the open distalend to facilitate retaining a solid probe. The leaves 43 are intended toengage a corresponding collar portion (not shown) of a probe 36 insertedthereinto.

Another embodiment of the invention is shown in FIG. 11 wherein aplurality of twist probes are configured in an array to form a TwinAccess Connector (TACC) module for interfacing test components with apersonality board. A testing device 100 includes a plurality of testcards 102 housed in a card cage. Each test card 102 has attached, to afront plate thereof, an interconnect adaptor 104. The details of theinterconnect adaptor can be found in allowed U.S. patent application,Ser. No. 07/585,800, filed Sep. 21, 1990, incorporated herein byreference. A rear TAC module 110 is attached to the front ofinterconnect adaptor 104, the combination being located by receiverframe 106. TAC module 110 includes a plurality of twist probes 118providing electrical contact between terminal ends 109 of cables 108 anda personality board 114. An opposite face of personality board 114engages twist probe connectors 112 of front TAC module 116 to provideelectrical conductivity to connector 122. Device Under Test (DUT) 124includes corresponding connectors to engage connector 122 and issupported by support plate 126.

The TAC modules shown in FIG. 11 permit rapid removal and replacement ofpersonality board 114 to adapt testing equipment 110 to various DUTs124. In particular, to change a personality board, receiver 130 isdisengaged, thereby releasing the ITA 120 which contains front TACmodule 116 and personality board 114. Upon minor disassembly of ITA 120the personality board 114 can then be removed and replaced by a newpersonality board and ITA 120 can be reassembled. Upon engaging receiver130, TAC module 116 is brought back into engagement with personalityboard 114. Because twist probes 118 rotate upon depression, oxide onconnector pads and components of personality board 114 is removed,thereby creating a low resistance connection.

Mounting of a double-headed twist probe 20 in a TAC module 110 is shownin greater detail in FIG. 12. Housing 22 is inserted into an aperture112 in frame 130 of module 110 until press ring 28 engages a frontsurface of the frame. Probe 20 is deformably retained in the aperture asshown. The frame may comprise an insulating substrate such as plasticwith an array of through holes for receiving probes 20. Peripheralportions of frame 130 include mounting structures for securing the frameto the front of a card cage.

Another embodiment of the invention illustrated in FIGS. 13 and 14incorporates a plurality of double-headed probes to directly interface apersonality board to a circuit board under test. The probes may bedouble-headed helix twist probes. Referring to FIG. 13, an interfacefixture 200 includes parallel top plate 202 and alignment plate 204.Plates 202 and 204 are made of a suitable electrical insulating materialsuch as plastic with a plurality of aligned through holes. Probes 118are positioned between the boards with opposite ends of the probesextending through respective vertically aligned through holes of plates202 and 204. The through holes have diameters greater than a housingdiameter of probes 118 positioned therein but less than the diameter ofretaining rings 28a and 28b formed proximate opposite ends of thehousings. Upward axial translation of probes 118 through the throughholes is constrained by engagement of upper retaining ring 28a bysurrounding portions of top plate 202 and downward translation islimited by lower retaining ring 28b engaging surrounding portions ofalignment plate 204.

Fixture 200 is positioned above a personality board 114 so that lowerexternal probe portions 36 of probes 118 engage contact pads formed onan upper surface of the personality board. Spring plungers 210 extendupward from personality board 114 and are retained within springhousings 212 provided at peripheral portions of the fixture. Returncompression springs 214 are seated atop spring plungers 210 withopposite ends of the springs engaging top plate 202 to bias top plate202 and alignment plate 204 upward. Upward travel of top plate 202 islimited by engagement of the head portions of spring plungers 210 withlower necked portions of spring housings 212.

Guide pins 216 and guide bushings 218 maintain alignment between thepersonality board 114, fixture 200 and a printed circuit board 230 of aunit under test (UUT). The guide pins 216 are positioned at peripheralportions of personality board 114 to engage corresponding guide bushingsextending through top plate 202 and alignment plate 204 to engage UUTgasket 220. Peripheral portions of printed circuit board 230 rest on UUTgasket 220 to position the printed circuit board parallel to and abovetop plate 202.

A frame member 222 is positioned atop personality board 114 with fixture200 and printed circuit board 230 located within the frame opening. Alower surface of frame member 222 includes a fixture gasket 224 made ofa resilient material. The gasket provides a compressible air sealbetween frame member 222 and underlying personality board 114. An uppersurface of frame member 222 includes a flange on which UUT gasket 220 isseated.

The combination of personality board 114, frame member 222, fixture 200and printed circuit board 230 form a closed chamber. When the chamber issubjected to atmospheric pressure as shown in FIG. 13 frame member 222,fixture 200 and printed circuit board 230 resting thereon are biasedupward, away from personality board 114 by return spring 114. In this"free state", component leads 232 and printed circuit pads 234 locatedon the lower surface of printed circuit board 230 are spaced abovepersonality board 114 whereby the upper contact terminals of twistprobes 118 are spaced apart from printed circuit board 230. Uponapplication of a vacuum source to the chamber, printed circuit board 230is drawn down under atmospheric pressure into engagement with twistprobes 118 as shown in FIG. 14. Alternate mechanical activation of thefixture is possible.

In summary, the double-headed twist probe according to the inventionprovides direct electrical conductivity between circuitry mounted onparallel substrates, avoiding intermediate connectors and wiring. Bymounting a plurality of double-headed twist probes in a suitablesupporting member, a double-sided "bed of nails" configuration isachieved to form a TAC module. The TAC module accommodates simplifiedremoval and replacement of frequently changed circuit board mountedcomponents, such as testing equipment personality boards.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

While there have been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims.

We claim:
 1. An electrical probe comprising:a housing having first andsecond opposite open ends; a first elongated plunger partiallypositioned within said housing said plunger rotatable about and slidablealong its longitudinal axis, within said housing and having(i) a firstportion extending out of said housing through said first opening end andterminating in a first electrical contact probe and (ii) a rod-likesecond portion lengthwise contiguous with said first portion positionedwithin said housing, said second portion having a bearing surface fortransmitting a torque; a second elongated plunger partially positionedwithin said housing, said second plunger rotatable about, and slidablealong, its axis, within said housing, and having(i) a first portionextending out from said housing through said second open end andterminating in a second electrical contact probe head and (ii) a tubularsecond portion lengthwise contiguous with said first portion, positionedwithin said housing, said second portion of said first elongated plungerpositioned within the second portion of said second elongated plunger,said second portion of said second elongated plunger having an aperturefor engaging said bearing surface of said first elongated plunger; and acompression spring positioned within said housing and engaging andaxially biasing said first and second elongated plungers outward fromsaid housing wherein said first and second opposite ends of said housinghaving restricted apertures for limiting axial travel of said first andsecond elongated plungers outwardly from said housing; said apertureincluding a keyway and second portion of said first elongated plungercomprises a cylindrical solid member having a helical channel formingsaid bearing surface for engaging said keyway, said keyway being formedat an obtuse angle to the longitudinal axis thereof to engage saidchannel during relative rotational movement of said plungers.
 2. Theprobe of claim 1 wherein said keyway is disposed at an angle of about174° to the longitudinal axis of said plunger.
 3. The probe assemblyaccording to claim 2 wherein said helical channel subtends a radialangle of between 90° and 150° over said length of said second portion ofsaid elongated plunger.
 4. The probe assembly according to claim 3wherein said channel subtends a radial angle of approximately 120° oversaid length of said second portion of said first elongated plunger. 5.The electrical probe assembly of claim 2 further comprising a secondhelical channel formed opposite said first helical channel in saidcylindrical solid member and a second keyway formed in said aperture ofsaid second plunger, said keyways respectively engaging the bearingsurfaces of said channels.
 6. The electrical probe assembly of claim 2further comprising an elongated leafspring formed in the tubular secondportion of said second plunger adjacent the aperture for engaging therod-like second portion of said first elongated plunger against lateralmovement during rotational slidable movement along the axis thereof. 7.The electrical probe assembly of claim 1 wherein the portion of saidsecond elongated plunger positioned outside said housing and terminatingin an electrical contact probe head further comprises a plurality ofinwardly directed leaves extending therewithin for engaging a contactprobe head inserted therewithin.
 8. An elongated electrical probeassembly comprising:a housing having an elongate tubular outer wallsurrounding a hollow interior space; a first plunger disposed in saidbarrel and rotatable about, and slidable along, its axis, within saidbarrel, and having an outer portion extending through an open end of thebarrel and terminating in a contact tip outside the barrel, said firstplunger including a cylindrical rod portion disposed within saidinterior of said barrel, said rod portion having an elongated controlsurface of helical configuration with an axial pitch and polygon-shapedcross section; a second plunger disposed in said barrel and rotatableabout, and slidable along, its axis, within said barrel, and having anouter portion extending through an opposite open end of the barrel andterminating in a contact tip outside the barrel, said second plungerincluding a hollow tubular portion disposed within said interior of saidbarrel, said hollow tubular portion having a polygon-shaped pilot holethrough which said cylindrical rod portion of said first plunger passeswhereby said pilot hole engages said engaging said control surface ofsaid rod portion; and a spring disposed in the barrel and biased againstthe first and second plungers so that axial travel of the plungers intothe barrel is against the bias of the spring, wherein said first one ofsaid plungers includes a portion which comprises a cylindrical solidmember having a helical channel forming a bearing surface for engagingsaid keyway, said keyway being formed at an obtuse angle to thelongitudinal axis thereof to engage said helical channel during relativerotational movement of said plungers.
 9. A double-headed spring contactprobe for loaded board testing, comprising:a barrel having a hollowinterior; a pair of opposite plungers positioned and axially slidableand rotatable in said barrel, said plungers having outer portionsextending through respective opposite open ends of the barrel, eachplunger terminating in a contact tip outside the barrel, a first one ofsaid plungers having a hollow receptacle extending into the barrel witha rectangular keyway opening into the receptacle, the other plungerhaving a twisted guide member extending through the barrel into thekeyway of the other plunger whereby axial translation of the plungersrelative to each other causes a rotation thereof; and a spring engagingopposite collar portions of the plungers, biasing the plungers outwardlyagainst opposite ends of the barrel, said barrel including neckedportions limiting travel of the plungers out from the barrel, whereinsaid first one of said plungers includes a portion which comprises acylindrical solid member having a helical channel forming a bearingsurface for engaging said keyway, said keyway being formed at an obtuseangle to the longitudinal axis thereof to engage said helical channelduring relative rotational movement of said plungers.